1. Field of the Invention
CORROSION--The present invention relates to a method for inhibiting the corrosion of metallic surfaces, especially those made of carbon steel, in contact with aqueous systems, and to compositions for use in such a method, particularly where the water of the aqueous system is oxygen-bearing. More particularly, the present invention relates to a synergistic composition comprising about 3 parts by weight of orthophosphate and about 1 part by weight of sodium silicate. In particular, sodium phosphate tribasic is the preferred form of orthophosphate.
The term "aqueous system" as used herein, is intended to describe any system which contains water in any physical state, including water which contains one or more dissolved or dispersed substances such as inorganic salts. Typical systems include, but are not limited to, cooling water systems including cooling towers, boiler water systems, desalination systems, gas scrubber units, blast furnaces, sewage sludge dewatering systems, thermal conditioning equipment, reverse osmosis units, sugar evaporators, paper processing systems, mining circuits, and the like. Where the aqueous system is a potable water source, it may be any type of drinking water system or source.
The term "carbon steel" as used herein is intended to include ferrous and ferrous-containing materials alloyed with small quantities of carbon and optionally small amounts of other metals, but to exclude those steel alloys of the type commonly referred to as "stainless", which contain nickel and/or chromium.
The corrosion of a metallic surface in an aqueous system consists of the destruction of the ferrous metal by chemical or electrochemical reaction of the metal with its immediate environment.
Where the corrosion is electrochemical in nature, a transfer or exchange of electrons is necessary for the corrosion reaction to proceed. When corrosion of the metal takes place, at least two electrochemical processes occur, and must occur, simultaneously. There is an anodic oxidation reaction in which metal ions go into solution, leaving behind electrons; and at least one cathodic reduction reaction in which species in solution are reduced by consuming the electrons produced by the anodic reaction. With respect to ferrous or ferrous containing materials, when the water contains oxygen and is at a neutral pH or above, these processes may be illustrated by the following equations:
Anodic oxidation: EQU Fe.fwdarw.Fe.sup.+2 +2e.sup.-.
Cathodic reaction: EQU 2H.sub.2 O+O.sub.2 +4e.sup.- .fwdarw.4H.sup.-.
The two ionic reaction products, ferrous ion and hydroxyl ion, combine to form ferrous hydroxide, Fe(OH).sub.2, which is then oxidized to form ferric hydroxide, Fe(OH).sub.3 (rust). For ferrous or ferrous-containing materials as well as other metals in aqueous systems, the principle factors influencing the corrosion process are the characteristics of the water in the system, including but not limited to the rate of water flow, the temperature of the system and contact between dissimilar metals in the system. Variable characteristics of the water which impact upon its corrosiveness are its dissolved oxygen concentration, carbon dioxide contant, pH and hardness.
The presence of dissolved oxygen in the water of an aqueous system is primarily the result of contact between the water and the atmosphere. The oxygen solubility in water is temperature and pressure dependent, with increases in pressure increasing solubility and increases in temperature lowering oxygen solubility.
Corrosion produced by the presence of oxygen in the water of an aqueous system can take place in the form of small pits or depressions and/or in the form of general metal loss. As a corrosive process continues, pits or depressions generally increase in depth. The corrosive attack is more severe when it causes pits or depressions, since the deeper penetration of the metal causes more rapid failure at these points.
MANGANESE AND IRON STABILIZATION--The synergistic combination of the present invention is also useful in stabilizing soluble manganese and iron ions and their reaction products in desirable forms and reduced particle sizes. Manganous ions are often found in well waters while cooling waters contain primarily the manganic species. Ferrous and ferric ions are often found in well waters while cooling waters contain primarily the ferric species. Anionic species of carbonate, bicarbonate, sulfite, fluoride, chloride, sulfate, and so forth, and dissolved oxygen may be present in both waters. Oxygen reaction products of manganese and iron can collect on metal surfaces and accelerate corrosion and reduce heat transfer.
LEAD LEACHING--The synergistic combination of the present invention is further useful for preventing or reducing levels of lead in potable water sources, i.e., drinking water. It has long been known that there is a strong link between lead contamination in drinking water and adverse health effects in humans.
2. Brief Description of the Prior Art
A variety of compositions have been employed in the art for the purpose of inhibiting corrosion of surfaces in water-carrying systems where the cause of corrosion is dissolved oxygen. Polyphosphates such as sodium tripolyphophate are widely used in the treatment of once-through systems. See U.S. Pat. No. 2,742,369. Silicates, for example sodium silicate, have also found acceptance.
U.S. Pat. No. 3,483,133 discloses a corrosion inhibiting composition comprising amino-tris (methylene phosphonic) acid compounds in combination with water soluble zinc salts.
Other conventional inhibitors such as zinc, soluble zinc salts, chromates, benzotriazole, tolytriazole or mercaptobenzothiazole may be added to the final formulation in varying amounts to improve its usefulness in a wider variety of industrial applications where both low carbon steel and copper or its alloys are present in the same system. Similarly, polymeric dispersants such as polyacrylates, polyacrylamides or polymers of 2-acrylamidomethylpropane sulfonic acid may also be incorporated in the final formulation in varying amounts. The molecular weights of these dispersants may vary from as low as less than 1000 to as high as several million.
Still other compositions for inhibiting corrosion are known. See, e.g., Boffardi U.S. Pat. No. 4,798,683 (molybdate compositions); Ralston U.S. Pat. No. 3,589,858 (readily soluble phosphate glasses); Hollingshad U.S. Pat. No. 3,885,914 (low molecular weight polymers and zinc); Ralston U.S. Pat. No. 4,018,701 (phosphorous acid and zinc); Hatch U.S. Pat. No. 3,532,639 (zinc salts and derivatives of methanol phosphonic or diphosphonic acid); and Hatch U.S. Pat. No. 3,022,133 (chromates/dichromates and zinc).
One method for removing soluble manganese by precipitation and removal involves the addition of a salt of iron, copper, or cobalt and any compound yielding bisulfite ions in solution to the manganese-containing water. See Hatch--U.S. Pat. No. 3,349,031.
Soluble manganese ion and its reaction products have been stabilized in water systems using carboxylic acid/sulphonic acid copolymers. See Ralston--U.S. Pat. No. 4,552,665.
The use of orthophosphate has reduced lead solubility in both low- and high-alkalinity waters. An orthophosphate concentration of approximately 1 to 2 mg/L PO.sub.4 can be effective in reducing lead solubility over a much lower pH range than would be possible by using pH-carbonate adjustment.
Adding zinc/polyphosphate to municipal distribution systems has been an effective treatment program for controlling corrosion, as well as stabilizing iron and manganese. Although polyphosphates are not as effective as orthophosphate in reducing lead solubility, the use of zinc/polyphosphate has broad applicability. The effective pH range is 6 to 7.5, but maintaining the pH above neutral is recommended.
Treatments utilizing silicates appear to have a retarding effect on lead solubility, but require a relatively long period of time, approximately 8 to 9 months, to show reductions in lead concentrations. This long-term effect can be explained by the slow formation of a kinetically-inhibited lead silicate film. Silicate treatments, however, are not recommended for control of lead solubility in distribution systems.